Discovery of rubber vulcanization almost two centuries ago gave rise to a continued interest in the use and recycling of cured rubber. Vulcanization is a chemical process for improving rubber elasticity and strength by heating thereof in the presence of sulfur. Sulfur is the oldest vulcanization agent, which links rubber polymer molecules together thus forming a three-dimensional network of individual polymeric chains cross-linked by sulfur bonds or sulfur bridges. During devulcanization process aforesaid sulfur-to-sulfur bonds are broken and the rubber particles are preferably returned to its reactive state.
Enormous numbers of used tyres, conveyor belts, hosepipes and other items are discarded each year. According to some estimates, annual production of rubber items reaches massive amount of 21 million tons, which corresponds to eight billion car tyres, for instance. These used rubber products typically end up at a landfill since there is no or very little use for them after the intended life-span.
As the demand for rubber in world market is extremely high and seems to remain that way, recycling of used rubber materials should be in the interest of both industrial users and environmentalists to secure the availability of the raw material and minimize e.g. the obvious waste problems associated with the rather challenging waste management of the used rubber products, respectively.
The existing rubber reclaiming and regenerating processes do not exploit devulcanization of rubber feedstock to the extent that would make a clear difference in the demand of new raw rubber eligible for vulcanization. The reason resides in the low yield and inferior quality, complexity, and obviously high price of the available devulcanization technology. Various microwave or ultrasound based methods, temperature control or temperature change based methods, biological methods, chemical methods such as oil-based methods, mechanical methods, and different combinations of the above have previously been suggested for use in devulcanization processes without considerable success. Devulcanization of discarded, cured rubber still remains a challenging problem, largely because during vulcanization the cross-linked rubber becomes thermoset and cannot be reformed into other products.
Polymeric molecular backbone created by vulcanization comprises apart the aforesaid sulfur bridges, also carbon-sulfur and carbon-carbon bonds, with characterizing binding energies of about 271.8 KJ (440 nm), 301 KJ (397 nm) and 347 KJ (345 nm), respectively. Known devulcanization processes rely on a wellknown fact that the energies of S—S and C—S bonds in the cured rubber network are lower than that of C—C bonds; therefore the former two types of bonds can be selectively broken up by various above mentioned methods while the parallel chains of molecular backbone are not influenced. However, the above-mentioned selective breakage has turned out to be a difficult objective. Since the mentioned binding energies are relatively close to each other, a de-crosslinking process of high selectivity is required. Implementation of such processes in practice by current devulcanization techniques remains problematic.